I assume once we migrate to Python3, we want to support Python 3.6-9.
Python 3.5 will stop receiving security fixes next month, so I don't
think we need to support it.
As running the test as many times as we want is free, run them for all
supported versions to ensure capa work in all of them.
Add the tests to the CI GitHub action. Only run the tests if code style
and rule linter succeed, as tests take some time to run.
A similar actions should be added to `capa-testfiles`.
